Fluorescent protein sensors for detection of analytes

ABSTRACT

Fluorescent indicators including a binding protein moiety, a donor fluorescent protein moiety, and an acceptor fluorescent protein moiety are described. The binding protein moiety has an analyte-binding region which binds an analyte and causes the indicator to change conformation upon exposure to the analyte. The donor moiety and the acceptor moiety change position relative to each other when the analyte binds to the analyte-binding region. The donor moiety and the acceptor moiety exhibit fluorescence resonance energy transfer when the donor moiety is excited and the distance between the donor moiety and the acceptor moiety is small. The indicators can be used to measure analyte concentrations in samples, such as calcium ion concentrations in cells.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Grant No. NS27177, awarded by the National Institutes of Health. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

The invention relates to fluorescent protein sensors for detecting and quantifying analytes.

Measurement of an analyte concentration in vitro or in vivo by non-invasive techniques can help elucidate the physiological function of the analyte. This can also aid in identifying changes that occur in a cell or organism in response to physiological stimuli. For example, cyclic AMP can be detected by fluorescence resonance energy transfer between a separately labeled proteins that associate with each other but are not covalently attached to each other. See, U.S. Pat. No. 5,439,797.

For example, many effects of Ca²⁺ in cells are mediated by Ca²⁺ binding to calmodulin (CaM), which causes CaM to bind and activate target proteins or peptide sequences. Based on the NMR solution structure of CaM bound to the 26-residue M13 Ca²⁺-binding peptide of myosin light-chain kinase, Porumb et al. fused the C-terminus of CaM via a Gly-Gly spacer to the M13. Ca²⁺ binding switches the resulting hybrid protein (CaM-M13) from a dumbbell-like extended form to a compact globular form similar to the CaM-M13 intermolecular complex. See, Porumb, T., et al., Prot. Engineering 7:109-115 (1994).

Fluorescent Ca²⁺ indicators such as fura-2, indo-1, fluo-3, and Calcium-Green have been the mainstay of intracellular Ca²⁺ measurement and imaging. See, for example, U.S. Pat. No. 4,603,209 and U.S. Pat. No. 5,049,673. These relatively low molecular weight indicators can suffer from many technical problems relating to ester loading, leakage of the dyes from the cell, compartmentation in organelles, and perturbation of the indicators by cellular constituents. Although the Ca²⁺-indicating photoprotein aequorin is targetable, the photoresponse to Ca²⁺ is low since it is chemiluminescent. Moreover, aequorins need to incorporate exogenous coelenterazine.

SUMMARY OF THE INVENTION

This invention provides fluorescent indicators and methods for using them to determine the concentration of an analyte both in vitro and in vivo. In one aspect, the fluorescent indicator includes a binding protein moiety, a donor fluorescent protein moiety, and an acceptor fluorescent protein moiety. The binding protein moiety has an analyte-binding region which binds an analyte and causes the indicator to change conformation upon exposure to the analyte. The donor fluorescent protein moiety is covalently coupled to the binding protein moiety. The acceptor fluorescent protein moiety is covalently coupled to the binding protein moiety. In the fluorescent indicator, the donor moiety and the acceptor moiety change position relative to each other when the analyte binds to the analyte-binding region, altering fluorescence resonance energy transfer between the donor moiety and the acceptor moiety when the donor moiety is excited.

The donor fluorescent protein moiety and the acceptor fluorescent protein moiety can be Aequorea-related fluorescent protein moieties. Preferably, the donor fluorescent protein moiety is P4-3, EBFP, or W1B, and the acceptor fluorescent protein moiety is S65T, EGFP, or 10c.

In preferred embodiments, the indicator further includes the target peptide moiety and a linker moiety that covalently couples the binding protein and the target peptide moiety. The binding protein moiety further includes a peptide-binding region for binding the target peptide moiety. The binding protein moiety can be covalently coupled to the donor fluorescent protein moiety and the target peptide moiety can be covalently coupled to the acceptor fluorescent protein moiety.

The indicator can be a single polypeptide. In preferred embodiments, one of the donor fluorescent protein moiety or the acceptor fluorescent protein moiety is covalently coupled to the carboxy terminus of the single polypeptide and the other of the donor fluorescent protein moiety or the acceptor fluorescent protein moiety is covalently coupled to the amino terminus of the single polypeptide.

The indicator can include a localization sequence. The localization sequence can be a nuclear localization sequence, an endoplasmic reticulum localization sequence, a peroxisome localization sequence, a mitochondrial import sequence, a mitochondrial localization sequence, or a localized protein.

In preferred embodiments, the linker moiety is a peptide moiety. The linker moiety can include between about 1 amino acid residue and about 20 amino acid residues. The linker moiety can be -Gly-Gly-.

Preferably, the binding protein moiety is calmodulin, a calmodulin-related protein moiety, cGMP-dependent protein kinase, a steroid hormone receptor, a ligand binding domain of a steroid hormone receptor, protein kinase C, inositol-1,4,5-triphosphate receptor, or recoverin. A calmodulin-related protein moiety is derived from calmodulin that has been modified to have a different binding affinity for calcium or a target peptide moiety.

Most preferably, the binding protein moiety is calmodulin or a calmodulin-related protein moiety. In these embodiments, the target peptide moiety can be a subsequence of a calmodulin-binding domain of M13, smMLCKp, CaMKII, Caldesmon, Calspermin, Calcineurin, PhK5, PhK13, C28W, 59-kDa PDE, 60-kDa PDE, NO-30, AC-28, Bordetella pertussis AC, Neuro-modulin, Spectrin, MARCKS, F52, β-Adducin, HSP90a, HIV-1 gp160, BBMHBI, Dilute MHC, Mastoparan, Melittin, Glucagon, Secretin, VIP, GIP, or Model Peptide CBP2. Preferably, the target peptide moiety is M13.

In another aspect, the invention features a fluorescent indicator including a target peptide moiety, a binding protein moiety, a linker moiety, a donor fluorescent protein moiety covalently coupled to the binding protein moiety, and an acceptor fluorescent protein moiety covalently coupled to the binding protein moiety. The binding protein moiety has an analyte-binding region which binds an analyte and causes the indicator to change conformation upon exposure to the analyte. The linker moiety covalently couples the binding protein and the target peptide moiety and is a peptide moiety. The binding protein moiety has a peptide-binding region for binding the target peptide moiety. The indicator is a single polypeptide.

In another aspect, the invention features a method for determining the concentration of an analyte in a sample. The method includes the steps of contacting the sample with a fluorescent indicator having a donor fluorescent protein moiety, binding protein moiety, and acceptor protein moiety, exciting the donor moiety, and determining the degree of fluorescence resonance energy transfer in the sample corresponding to the concentration of the analyte in the sample.

In preferred embodiments, the step of determining the degree of fluorescence resonance energy transfer in the sample includes measuring light emitted by the acceptor fluorescent protein moiety. In other preferred embodiments, determining the degree of fluorescence resonance energy transfer in the sample includes measuring light emitted from the donor fluorescent protein moiety, measuring light emitted from the acceptor fluorescent protein moiety, and calculating a ratio of the light emitted from the donor fluorescent protein moiety and the light emitted from the acceptor fluorescent protein moiety. In yet other preferred embodiments, determining the degree of fluorescence resonance energy transfer in the sample includes measuring the excited state lifetime of the donor moiety.

The method can further include the steps of determining the concentration of the analyte at a first time after contacting the sample with the fluorescence indicator, determining the concentration of the analyte at a second time after contacting the sample with the fluorescence indicator, and calculating the difference in the concentration of the analyte at the first time and the second time, whereby the difference in the concentration of the analyte in the sample reflects a change in concentration of the analyte present in the sample.

In other embodiments, the method can further include the step of contacting the sample with a compound between the first time and the second time, whereby a difference in the concentration of the analyte in the sample between the first time and the second time indicates that the compound alters the presence of the analyte.

In preferred embodiments, the sample includes an intact cell and the contacting step includes incorporating the fluorescent indicator into the cell. The step of incorporating the fluorescent indicator into the cell can include transfecting the cell with an expression vector comprising expression control sequences operably linked to a nucleic acid sequence coding for the expression of the fluorescent indicator. The preferred analyte is calcium.

In yet another aspect, the invention features an isolated nucleic acid sequence which encodes the fluorescent indicator. In preferred embodiments, an expression vector or a transgenic non-human animal includes the nucleic acid sequence.

In another aspect, the invention features an expression vector including expression control sequences operatively linked to a nucleic acid sequence coding for the expression of the fluorescent indicator. The expression vector can be adapted for function in a prokaryotic cell or a eukaryotic cell.

In another aspect of the invention, a host cell transfected with an expression vector can include an expression control sequence operatively linked to a sequence coding for the expression of the fluorescent indicator. The cell can be a prokaryote, such as E. coli, or a eukaryotic cell, such as a yeast cell or mammalian cell.

In another aspect, the invention features a transgenic non-human animal having a phenotype characterized by expression of the nucleic acid sequence coding for the expression of the fluorescent indicator. The phenotype is conferred by a transgene contained in the somatic and germ cells of the animal. The animal can be a mouse.

In another aspect, the invention features a method for producing a transgenic non-human animal having a phenotype characterized by expression of the nucleic acid sequence coding for the expression of the fluorescent indicator. The method includes the steps of: (a) introducing a transgene into a zygote of an animal, the transgene comprising a DNA construct encoding the fluorescent indicator; (b) transplanting the zygote into a pseudopregnant animal; (c) allowing the zygote to develop to term; and (d) identifying at least one transgenic offspring containing the transgene. The step of introducing of the transgene into the embryo can be by introducing an embryonic stem cell containing the transgene into the embryo, or infecting the embryo with a retrovirus containing the transgene.

“Peptide” refers to a polymer in which the monomers are amino acid residues which are joined together through amide bonds, alternatively referred to as a polypeptide. A “single polypeptide” is a continuous peptide that constitutes the protein. When the amino acids are alpha-amino acids, either the L-optical isomer or the D-optical isomer can be used, the L-isomers being preferred. Additionally, unnatural amino acids such as beta-alanine, phenylglycine, and homo-arginine are meant to be included. Commonly encountered amino acids which are not gene-encoded can also be used in the present invention, although preferred amino acids are those that are encodable. For a general review, see, for example, Spatola, A. F., in Chemistry and Biochemistry of Amino Acids, Peptides and Proteins, B. Weinstein, ed., Marcel Dekker, N.Y., p. 267 (1983).

“Fluorescent protein” refers to any protein capable of emitting light when excited with appropriate electromagnetic radiation. Fluorescent proteins include proteins having amino acid sequences that are either natural or engineered, such as the fluorescent proteins derived from Aequorea-related fluorescent proteins.

In FRET, the “donor fluorescent protein moiety” and the “acceptor fluorescent protein moiety” are selected so that the donor and acceptor moieties exhibit fluorescence resonance energy transfer when the donor moiety is excited. One factor to be considered in choosing the donor/acceptor fluorescent protein moiety pair is the efficiency of FRET between the two moieties. Preferably, the efficiency of FRET between the donor and acceptor moieties is at least 10%, more preferably at least 50%., and most preferably at least 80%. The efficiency of FRET can be tested empirically using the methods described herein and known in the art, particularly, using the conditions set forth in the Examples.

“Covalently coupled” refers to a covalent bond or other covalent linkage between two moieties. The covalent linkage can include a diradical moiety linking to two moieties.

“Binding protein” refers to a protein capable of binding an analyte. Preferred binding proteins change conformation upon binding the analyte. “Target peptide” refers to a peptide that can bind to the binding protein. The target peptide can be a subsequence of a peptide that binds to the binding protein.

“Analyte” refers to a molecule or ion in solution that binds to the binding protein, causing it to change conformation. Preferably, the analyte binds reversibly to the binding protein.

“Moiety” refers to a radical of a molecule that is attached to another radical of the indicator. Thus, a “fluorescent protein moiety” is the radical of a fluorescent protein coupled to a binding protein moiety or a linker moiety, a “binding protein moiety” is a radical of a binding protein coupled to a fluorescent protein moiety, a “target peptide moiety” is a radical of a target peptide of the binding protein, and a “linker moiety” refers to the radical of a molecular linker that is ultimately coupled to both the donor and acceptor fluorescent protein moieties.

“Operatively linked” refers to a juxtaposition wherein the components so described are in a relationship permitting them to function in their intended manner. A control sequence operatively linked to a coding sequence is ligated such that expression of the coding sequence is achieved under conditions compatible with the control sequences. “Control sequence” refers to polynucleotide sequences which are necessary to effect the expression of coding and non-coding sequences to which they are ligated. Control sequences generally include promoter, ribosomal binding site, and transcription termination sequence. The term “control sequences” is intended to include, at a minimum, components whose presence can influence expression, and can also include additional components whose presence is advantageous, for example, leader sequences and fusion partner sequences.

“Polynucleotide” refers to a polymeric form of nucleotides of at least 10 bases in length. The nucleotides can be ribonucleotides, deoxynucleotides, or modified forms of either type of nucleotide. The term includes single and double stranded forms of DNA.

The invention can have one or more of the following advantages. Ligand-induced conformational changes can be monitored by FRET if, for example, the amino and carboxy termini of the binding protein are fused to a donor and acceptor GFP. This approach has several advantages over the usual covalent labeling with fluorescent probes. The fluorescent indicator can be generated in situ by gene transfer into the cells or organisms. This approach avoids the need to express and purify large quantities of soluble recombinant protein, purify and label it in vitro, microinject it back into cells. The fluorescent indicator can be targeted to cellular structures. The sites of fusion between the moieties of the fluorescent indicator are exactly defined, giving a molecularly homogenous product without relying on elaborate protein chemistry. In addition, the chromophore of GFP is fixed in the protein. See, Ormo, M., et al., Science 273:1392-1395 (1996). If the GFP donor and acceptor are fused to a host protein rigidly, minor changes in the relative orientation of the ends of the latter would alter FRET. In contrast, most conventional fluorescent labels are attached by flexible linkers that at least partially decouple the fluorophore orientation from that of the protein to which it is attached, limiting the sensitivity of the FRET measurement.

Other features or advantages of the present invention will be apparent from the following detailed description of the invention, and also from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram depicting a fluorescent indicator that measures the concentration of an analyte by fluorescence resonance energy transfer.

FIG. 2 a is a schematic diagram depicting the structure of chimera proteins containing P4-3, CaM-M13, and S65T, where HIS is the amino-terminal tag peptide containing the polyhistidine sequence and XCaM is Xenopus calmodulin.

FIG. 2 b is a diagram depicting the amino acid and nucleotide sequences of the boundaries between P4-3 and CaM and between M13 and S65T in cameleon-1.

FIG. 3 is a graph depicting the emission spectra of cameleon-1 before and after addition of 2 mM Cal₂ to give 1 mM free Ca²⁺.

FIG. 4 a is a graph depicting the change in emission spectrum of cameleon-1 on titration with Ca²⁺ when excited at 380 nm.

FIG. 4 b is a graph depicting Ca²⁺ titration curves of cameleon-1 (open circles) and cameleon-1/E104Q (solid circles).

FIG. 5 is a schematic diagram depicting the structures of cameleon-2 and cameleon-3, and their derivatives, cameleon-2nu and cameleon-3er, where Kz is Kozak's consensus sequence.

FIG. 6 a is a digital fluorescence image depicting fluorescence of cameleon-2 localized to the cytosol of HeLa cells, where the bar is 10 μm.

FIG. 6 b is a graph depicting temporal changes in the emission ratio of cameleon-2 for each of the HeLa cells shown in FIG. 6 a.

FIG. 6 c is a digital fluorescence image depicting fluorescence of cameleon-2nu localized to the nuclei of HeLa cells, where the bar is 10 μm.

FIG. 6 d is a graph depicting temporal changes in the emission ratio of cameleon-2nu in the two nuclei shown in FIG. 6 c.

FIG. 6 e is a digital fluorescence image depicting fluorescence of cameleon-3er in transfected HeLa cells, where the bar is 10 μm.

FIG. 6 f is a graph depicting the time-course of emission ratio of cameleon-3er (average of four cells).

FIG. 7 depicts the nucleotide sequence (SEQ ID NO:1) and amino acid sequence (SEQ ID NO:2) of cameleon-2.

FIG. 8 depicts the nucleotide sequence (SEQ ID NO:3) and amino acid sequence (SEQ ID NO:4) of cameleon-2nu.

FIG. 9 depicts the nucleotide sequence (SEQ ID NO:5) and amino acid sequence (SEQ ID NO:6) of cameleon-3.

FIG. 10 is a list depicting the nucleotide sequence (SEQ ID NO:7) and amino acid sequence of cameleon-3er (SEQ ID NO:8).

DETAILED DESCRIPTION

A fluorescent indicator that utilizes fluorescent resonance energy transfer (“FRET” ) to measure the concentration of an analyte includes two fluorescent protein moieties having emission and excitation spectra that render one a donor fluorescent protein moiety and the other an acceptor fluorescent protein moiety. The fluorescent protein moieties are chosen such that the excitation spectrum of one of the moieties (the acceptor fluorescent protein moiety) overlaps with the emission spectrum of the excited protein moiety (the donor fluorescent protein moiety). The donor and acceptor fluorescent protein moieties are bound to a binding protein moiety that changes conformation upon binding the analyte. The change in conformation leads to a change in relative position and orientation of the donor and acceptor fluorescent protein moieties, thereby altering the relative amounts of fluorescence from the two fluorescent protein moieties when the donor is excited by irradiation. In particular, binding of the analyte changes the ratio of the amount of light emitted by the donor and acceptor fluorescent protein moieties. The ratio between the two emission wavelengths provides a measure of the concentration of the analyte in the sample, which is based in part on the binding affinity of the binding protein moiety and the analyte.

Referring to FIG. 1, the donor fluorescent protein moiety is covalently linked to a first region (e.g., the amino terminus) of the binding protein moiety, and the acceptor fluorescent protein moiety is covalently linked to a second region (e.g., the carboxy terminus) of the binding protein moiety such that the donor and acceptor moieties move closer together upon binding the analyte. Alternatively, the donor and acceptor moieties can move farther apart upon binding the analyte. In one embodiment, depicted in FIG. 1, the acceptor moiety is covalently bonded to a target peptide moiety that also binds to the binding protein moiety and the target peptide moiety is covalently bonded to the binding protein moiety by a linker moiety. The linker moiety is flexible enough to allow the target peptide moiety to bind to the binding protein moiety. The donor moiety is excited by light of appropriate intensity within the excitation spectrum of the donor moiety (λ_(excitation)). The donor moiety emits the absorbed energy as fluorescent light (λ_(emission 1)). When the acceptor fluorescent protein moiety is positioned to quench the donor moiety in the excited state, the fluorescence energy is transferred to the acceptor moiety which can emit fluorescent light (λ_(emission 2)). FRET can be manifested as a reduction in the intensity of the fluorescent signal from the donor moiety (λ_(emission 1)), reduction in the lifetime of the excited state of the donor moiety, or emission of fluorescent light at the longer wavelengths (lower energies) characteristic of the acceptor moiety (λ_(emission 2)). When the conformation of the binding protein moiety changes upon binding the analyte, the fluorescent protein moieties come closer together (or physically separate), and FRET is increased (or decreased) accordingly.

The efficiency of FRET depends on the separation distance and the orientation of the donor and acceptor fluorescent protein moieties. For example, the Forster equation describes the efficiency of excited state energy transfer, based in part on the fluorescence quantum yield of the donor moiety and the energetic overlap with the acceptor moiety. The Forster equation is: E=(F₀−F)/F₀=R₀ ⁶/(R⁶+R₀ ⁶) where E is the efficiency of FRET, F and F₀ are the fluorescence intensities of the donor moiety in the presence and absence of the acceptor, respectively, and R is the distance between the donor moiety and the acceptor moiety.

The characteristic distance R₀ at which FRET is 50% efficient depends on the quantum yield of the donor moiety (i.e., the shorter-wavelength fluorophore), the extinction coefficient of the acceptor moiety (i.e., the longer-wavelength fluorophore), and the overlap between the emission spectrum of the donor moiety and the excitation spectrum of the acceptor moiety. R₀ is given (in Å) by R₀=9.79×10³(K²QJn⁻⁴)^(1/6) where K² is an orientation factor having an average value close to 0.67 for freely mobile donors and acceptors, Q is the quantum yield of the unquenched donor moiety, n is the refractive index of the medium separating the donor moiety and the acceptor moiety, and J is the overlap integral. J can be quantitatively expressed as the degree of spectral overlap between the donor moiety and the acceptor moiety according to the equation: J=∫^(∞) ₀ε_(λ)F_(λ)λ⁴dλ/∫^(∞) ₀F_(λ)dλ where ε_(λ)(M⁻¹cm⁻¹) is the molar absorptivity of the acceptor and F_(λ) is the donor moiety fluorescence intensity at wavelength λ. See, for example, Forster, T. Ann. Physik 2:55-75 (1948). Tables of spectral overlap integrals are readily available to those working in the field (for example, Berlman, I. B. Energy transfer parameters of aromatic compounds, Academic Press, N.Y. and London (1973)). FRET is a nondestructive spectroscopic method that can monitor proximity and relative angular orientation of fluorophores in living cells. See, for example, Adams, S. R., et al., Nature 349:694-697 (1991), and Gonzalez, J. & Tsien, R. Y. Biophy. J. 69:1272-1280 (1995).

These factors need to be balanced to optimize the efficiency and detectability of FRET from the fluorescent indicator. The emission spectrum of the donor fluorescent protein moiety should overlap as much as possible with the excitation spectrum of the acceptor fluorescent protein moiety to maximize the overlap integral J. Also, the quantum yield of the donor fluorescent protein moiety and the extinction coefficient of the acceptor fluorescent protein moiety should be as large as possible to maximize R₀. In addition, the excitation spectra of the donor and acceptor moieties should overlap as little as possible so that a wavelength region can be found at which the donor moiety can be excited selectively and efficiently without directly exciting the acceptor moiety. Direct excitation of the acceptor moiety should be avoided since it can be difficult to distinguish direct emission from fluorescence arising from FRET. Similarly, the emission spectra of the donor and acceptor moieties should have minimal overlap so that the two emissions can be distinguished. High fluorescence quantum yield of the acceptor moiety is desirable if the emission from the acceptor moiety is to be monitored to determine analyte concentration in a sample. In a preferred embodiment, the donor fluorescent protein moiety is excited by ultraviolet (<400 nm) and emits blue light (<500 nm), and the acceptor fluorescent protein moiety is efficiently excited by blue but not by ultraviolet light and emits green light (>500 nm), for example, P4-3 and S65T, respectively.

In another preferred embodiment, the donor fluorescent moiety is excited by violet (400-430 nm) and emits blue-green (450-500 nm) and the acceptor fluorescent moiety is efficiently excited by blue-green (450-500 nm) and emits yellow-green light (≧520 nm), for example WIB and 10C respectively.

The amount of analyte in a sample can be determined by determining the degree of FRET in the sample. Changes in analyte concentration can be determined by monitoring FRET at a first and second time after contact between the sample and the fluorescent indicator and determining the difference in the degree of FRET. The amount of analyte in the sample can be calculated by using a calibration curve established by titration.

The degree of FRET can be determined by any spectral or fluorescence lifetime characteristic of the excited donor moiety. For example, intensity of the fluorescent signal from the donor, the intensity of fluorescent signal from the acceptor, the ratio of the fluorescence amplitudes near the acceptor's emission maxima to the fluorescence amplitudes near the donor's emission maximum, or the excited state lifetime of the donor can be monitored.

Preferably, changes in the degree of FRET are determined as a function of the change in the ratio of the amount of fluorescence from the donor and acceptor moieties, a process referred to as “ratioing.” Changes in the absolute amount of indicator, excitation intensity, and turbidity or other background absorbances in the sample at the excitation wavelength affect the intensities of fluorescence from both the donor and acceptor approximately in parallel. Therefore the ratio of the two emission intensities is a more robust and preferred measure of cleavage than either intensity alone.

Fluorescence in a sample is measured using a fluorometer. In general, excitation radiation, from an excitation source having a first wavelength, passes through excitation optics. The excitation optics cause the excitation radiation to excite the sample. In response, fluorescent proteins in the sample emit radiation which has a wavelength that is different from the excitation wavelength. Collection optics then collect the emission from the sample. The device can include a temperature controller to maintain the sample at a specific temperature while it is being scanned. According to one embodiment, a multi-axis translation stage moves a microtiter plate holding a plurality of samples in order to position different wells to be exposed. The multi-axis translation stage, temperature controller, auto-focusing feature, and electronics associated with imaging and data collection can be managed by an appropriately programmed digital computer. The computer also can transform the data collected during the assay into another format for presentation.

Methods of performing assays on fluorescent materials are well known in the art and are described in, e.g., Lakowicz, J. R., Principles of Fluorescence Spectroscopy, New York:Plenum Press (1983); Herman, B., Resonance energy transfer microscopy, in: Fluorescence Microscopy of Living Cells in Culture, Part B, Methods in Cell Biology, vol. 30, ed. Taylor, D. L. & Wang, Y.-L., San Diego: Academic Press (1989), pp. 219-243; Turro, N. J., Modern Molecular Photochemistry, Menlo Park: Benjamin/Cummings Publishing Col, Inc. (1978), pp. 296-361.

The excited state lifetime of the donor moiety is, likewise, independent of the absolute amount of substrate, excitation intensity, or turbidity or other background absorbances. Its measurement requires equipment with nanosecond time resolution.

Quantum yields of wild-type GFP, S65T, and P4-1 mutants can be estimated by comparison with fluorescein in 0.1 N NaOH as a standard of quantum yield 0.91. J. N. Miller, ed., Standards in Fluorescence Spectrometry, New York: Chapman and Hall (1981). Mutants P4 and P4-3 were likewise compared to 9-aminoacridine in water (quantum yield 0.98).

Any fluorescent protein can be used in the invention, including proteins that fluoresce due intramolecular rearrangements or the addition of cofactors that promote fluorescence. For example, green fluorescent proteins of cnidarians, which act as their energy-transfer acceptors in bioluminescence, are suitable fluorescent proteins for use in the fluorescent indicators. A green fluorescent protein (“GFP”) is a protein that emits green light, and a blue fluorescent protein (“BFP”) is a protein that emits blue light. GFPs have been isolated from the Pacific Northwest jellyfish, Aequorea victoria, the sea pansy, Renilla reniformis, and Phialidium gregarium. See, Ward, W. W., et al., Photochem. Photobiol., 35:803-808 (1982); and Levine, L. D., et al., Comp. Biochem. Physiol., 72B:77-85 (1982).

A variety of Aequorea-related GFPs having useful excitation and emission spectra have been engineered by modifying the amino acid sequence of a naturally occurring GFP from Aequorea victoria. See, Prasher, D. C., et. al., Gene, 111:229-233 (1992); Heim, R., et al., Proc. Natl. Acad. Sci., USA, 91:12501-04 (1994); U.S. Ser. No. 08/337,915, filed Nov. 10, 1994; International application PCT/US95/14692, filed Nov. 10, 1995; and U.S. Ser. No. 08/706,408, filed Aug. 30, 1996. The cDNA of GFP can be concatenated with those encoding many other proteins; the resulting fusions often are fluorescent and retain the biochemical features of the partner proteins. See, Cubitt, A. B., et al., Trends Biochem. Sci. 20:448-455 (1995). Mutagenesis studies have produced GFP mutants with shifted wavelengths of excitation or emission. See, Heim, R. & Tsien, R. Y. Current Biol. 6:178-182 (1996). Suitable pairs, for example a blue-shifted GFP mutant P4-3 (Y66H/Y145F) and an improved green mutant S65T can respectively serve as a donor and an acceptor for fluorescence resonance energy transfer (FRET). See, Tsien, R. Y., et al., Trends Cell Biol. 3:242-245 (1993). A fluorescent protein is an Aequorea-related fluorescent protein if any contiguous sequence of 150 amino acids of the fluorescent protein has at least 85% sequence identity with an amino acid sequence, either contiguous or non-contiguous, from the wild type Aequorea green fluorescent protein. More preferably, a fluorescent protein is an Aequorea-related fluorescent protein if any contiguous sequence of 200 amino acids of the fluorescent protein has at least 95% sequence identity with an amino acid sequence, either contiguous or non-contiguous, from the wild type Aequorea green fluorescent protein. Similarly, the fluorescent protein can be related to Renilla or Phialidium wild-type fluorescent proteins using the same standards. Some Aequorea-related engineered versions described in Table I. Other variants or mutants are within the scope of the invention as described, for example, in the Examples. TABLE I Extinction Excitation Emission Coefficient Quantum Clone Mutation(s) max (nm) max (nm) (M⁻¹cm⁻¹) yield Wild type none 395 (475) 508 21,000 (7,150) 0.77 P4 Y66H 383 447 13,500 0.21 P4-3 Y66H; Y145F 381 445 14,000 0.38 W7 Y66W; N146I 433 (453) 475 (501) 18,000 (17,100) 0.67 M153T V163A N212K W2 Y66W; I123V 432 (453) 480 10,000 (9,600) 0.72 Y145H H148R M153T V163A N212K S65T S65T 489 511 39,200 0.68 P4-1 S65T; M153A 504 (396) 514 14,500 (8,600) 0.53 K238E S65A S65A 471 504 S65C S65C 479 507 S65L S65L 484 510 Y66F Y66F 360 442 Y66W Y66W 458 480 10c S65G; V68L 513 527 S72A; T203Y W1B F64L; S65T 432 (453) 476 (503) Y66W; N146I M153T V163A N212K Emerald S65T; S72A 487 508 N149K M153T I167T Sapphire S72A; Y145F 395 511 T203I An additional clone, W1B1 included the following mutations: F64L;S65T; Y66W; F99S; and V163A.

Other fluorescent proteins can be used in the fluorescent indicators, such as, for example, yellow fluorescent protein from Vibrio fischeri strain Y-1, Peridinin-chlorophyll a binding protein from the dinoflagellate Symbiodinium sp.phycobiliproteins from marine cyanobacteria such as Synechococcus, e.g., phycoerythrin and phycocyanin, or oat phytochromes from oat reconstructed with phycoerythrobilin. These fluorescent proteins have been described in Baldwin, T. O., et al., Biochemistry 29:5509-5515 (1990), Morris, B. J., et al., Plant Molecular Biology, 24:673-677 (1994), and Wilbanks, S. M., et al., J. Biol. Chem. 268:1226-1235 (1993), and Li et al., Biochemistry 34:7923-7930 (1995).

The efficiency of FRET between the donor and acceptor fluorescent protein moieties can be adjusted by changing ability of the two fluorescent proteins to closely associate. The nature of the binding protein moiety, target peptide moiety, and linker moiety each affect the FRET and the response of the indicator to the analyte. Generally, large conformational changes in the binding protein moiety are desired along with a high affinity for the target peptide moiety.

The binding protein moiety is a protein, or part thereof, that changes conformation upon binding an analyte. Proteins that undergo useful conformation change upon binding an analyte include calmodulin (CaM), cGMP-dependent protein kinase, steroid hormone receptors (or ligand binding domain thereof), protein kinase C, inositol-1,4,5-triphosphate receptor, or recoverin. See, for example, Katzenellenbogen, J. A. & Katzenellenbogen, B. S. Chemistry & Biology 3:529-536 (1996), and Ames, J. B., et al., Curr. Opin. Struct. Biol. 6:432-438 (1996). The binding protein moiety preferably binds target peptides in addition to the analyte. The Ca²⁺-binding affinity of calmodulin can be tuned as reviewed, for example, in Falke, J. J., et al., Quart. Rev. Biophys. 27:219-290 (1994).

The target peptide moiety can contain any of the amino acid sequences in Table II, or a portion thereof with the proviso that the target peptide must bind to the binding protein moiety. The target peptide can be a subsequence of a calmodulin-binding domain. The target peptide moieties listed in Table II are recognized by the binding protein moiety CaM. See, for example, Crivici, A. & Ikura, M. Annu. Rev. Biophys. Biomol. Struct. 24:84-116 (1995). The target peptide moiety can be modified to enhance the response of the fluorescent indicator to the analyte. Other target peptide moieties are known in the art for other binding proteins. TABLE II Target^(a) Sequence skMLCK (M13)        KRRWKKNFIAVSAANRFKKISSSGAL smMLCK (smMLCKp)       ARRKWQKTGHAVRAIGRLSS CaMKII       ARRKLKGAILTTMLATRNFS Caldesmon          GVRNIKSMWEKGNVFSS Calspermin       ARRKLKAAVKAVVASSRLGS PFK (M11)    FMNNWEVYKLLAHIRPPAPKSGSYTV Calcineurin      ARKEVIRNKIRAIGKMARVFSVLR PhK (PhK5)         LRRLIDAYAFRIYGHWVKKGQQQNRG   (PhK13)        RGKFKVICLTVLASVRIYYQYRRVKPG Ca²⁺-ATPase (C28W)    LRRGQILWFRGLNRIQTQIKVVNAFSSS 59-kDa PDE       RRKHLQRPIFRLRCLVKQLEK 60-kDa PDE       TEKMWQRLKGILRCLVKQLEK NOS (NO-30)     KRRAIGFKKLAEAVKFSAKLMGQ Type I AC (AC-28)   IKPAKRMKFKTVCYLLVQLMHCRKMFKA Borderella periussis AC         IDLLWKIARAGARSAVGTEA Neuromodulin   KAHKAATKIQASFRGHITRKKLKGEKK Spectrin      KTASPWKSARLMVHTVATFNSIKE MARCKS     KKKKKRFSFKKSFKLSGFSFKKSKK F52 or MacMARKS      KKKKKFSFKKPFKLSGLSFKRNRK β-Adducin   KQQKEKTRWLNTPNTYLRVNVADEVQRNMGS HSP90a        KDQVANSAFQERLRKHGLEVI HIV-1 gp160           YHRLRDLLLIVKRIVELLGRR BBMHBI     QQLATLIQKTYRGWRCRTHYQLM Dilute MHC     RAACIRIQKTIRGWLLRKRYLCMQ Mastoparan           INLKALAALAKKIL Melittin      GIGAVLKVLTTGLPALISWIKRKRQQ Glucagon HSQGTFTTSDYSKYLDSRRAQDFVQWLMNT Secretin  HSDGTFTSELSRLRDSARLQRLLQGLV VIP  HSDAVFTDNYTRLRKQMAVKKYLNSILN GIP  YADGTFISDYSAIMNKIRQQDFVNWLLAQQQKS Model Peptide CBP2         KLWKKLLKLLKKLLKLG ^(a)Abbreviations: AC, adenylyl cyclase; BBMHCI, brush-border myosin heavy chain-I; CaMKII, calmodulin kinase II; CBP2, calmodulin binding peptide-2; GIP, gastrin inhibitory peptide; HIV-1 gp160, human immunodeficiency virus envelope glycoprotein 160; HSP, heat-shock protein; MARCKS, myristoylated alaminte-rich C kinase substrate; MHC, myosin heavy chain; NOS, nitric oxide synthase; PDE, phosphodiesterase; PFK, phosphofructokinase; PhK, phosphorylase kinase; sk-, smMLCK, skeletal muscle- and smooth muscle-myosin light chain kinase; VIP, vasoactive intestinal peptide.

The length of the linker moiety is chosen to optimize both FRET and the kinetics and specificity of conformational changes induced by analyte binding. The linker moiety should be long enough and flexible enough to allow the binding protein moiety and target peptide moiety to freely interact and respond to analyte concentration. In order to optimize the FRET effect, the average distance between the donor and acceptor fluorescent protein moieties should become between about 1 nm and about 10 nm, preferably between about 1 nm and about 6 nm, and more preferably between about 1 nm and about 4 nm, when the analyte is bound (or released). If the linker is too short or too stiff, the donor and acceptor protein moieties cannot readily change position. If the linker moiety is too long, the target peptide moiety might not bind to the binding protein moiety effectively. The linker moiety is, preferably, a peptide moiety. The preferred linker moiety is a peptide between about one and 30 amino acid residues in length, preferably between about two and 15 amino acid residues. One preferred linker moiety is a -Gly-Gly-linker.

The linker moiety can include flexible spacer amino acid sequences, such as those known in single-chain antibody research. For example, the linker moiety can be GGGGS (GGGGS)_(n), GKSSGSGSESKS, GSTSGSGKSSEGKG, GSTSGSGKSSEGSGSTKG, GSTSGSGKSSEGKG, GSTSGSGKPGSGEGSTKG, or EGKSSGSGSESKEF. Linking moieties are described, for example, in Huston, J. S., et al., PNAS 85:5879-5883 (1988), Whitlow, M., et al., Protein Engineering 6:989-995 (1993), and Newton, D. L., et al., Biochemistry 35:545-553 (1996).

The fluorescent indicators can also include a localization sequence to direct the indicator to particular cellular sites by fusion to appropriate organellar targeting signals or localized host proteins. A polynucleotide encoding a localization sequence, or signal sequence, can be ligated or fused at the 5′ terminus of a polynucleotide encoding the fluorescence indicator such that the signal peptide is located at the amino terminal end of the resulting fusion polynucleotide/polypeptide. In the case of eukaryotes, the signal peptide is believed to function to transport the fusion polypeptide across the endoplasmic reticulum. The secretory protein is then transported through the Golgi apparatus, into secretory vesicles and into the extracellular space or, preferably, the external environment. Signal peptides which can be utilized according to the invention include pre-pro peptides which contain a proteolytic enzyme recognition site. Other signal peptides with similar properties to pro-calcitonin described herein are known to those skilled in the art, or can be readily ascertained without undue experimentation.

The localization sequence can be a nuclear localization sequence, an endoplasmic reticulum localization sequence, a peroxisome localization sequence, a mitochondrial localization sequence, or a localized protein. Localization sequences can be targeting sequences which are described, for example, in “Protein Targeting”, chapter 35 of Stryer, L., Biochemistry (4th ed.). W. H. Freeman, 1995. The localization sequence can also be a localized protein. Some important localization sequences include those targeting the nucleus (KKKRK), mitochondrion (amino terminal MLRTSSLFTRRVQPSLFRNILRLQST-), endoplasmic reticulum (KDEL at C-terminus, assuming a signal sequence present at N-terminus), peroxisome (SKF at C-terminus), prenylation or insertion into plasma membrane (CaaX, CC, CXC, or CCXX at C-terminus), cytoplasmic side of plasma membrane (fusion to SNAP-25), or the Golgi apparatus (fusion to furin).

The fluorescent indicators can be produced as fusion proteins by recombinant DNA technology. Recombinant production of fluorescent proteins involves expressing nucleic acids having sequences that encode the proteins. Nucleic acids encoding fluorescent proteins can be obtained by methods known in the art. For example, a nucleic acid encoding the protein can be isolated by polymerase chain reaction of cDNA from A. victoria using primers based on the DNA sequence of A. victoria green fluorescent protein. PCR methods are described in, for example, U.S. Pat. No. 4,683,195; Mullis, et al. Cold Spring Harbor Symp. Quant. Biol. 51:263 (1987), and Erlich, ed., PCR Technology, (Stockton Press, N.Y., 1989). Mutant versions of fluorescent proteins can be made by site-specific mutagenesis of other nucleic acids encoding fluorescent proteins, or by random mutagenesis caused by increasing the error rate of PCR of the original polynucleotide with 0.1 mM MnCl₂ and unbalanced nucleotide concentrations. See, e.g., U.S. patent application Ser. No. 08/337,915, filed Nov. 10, 1994 or International application PCT/US95/14692, filed Nov. 10, 1995.

The construction of expression vectors and the expression of genes in transfected cells involves the use of molecular cloning techniques also well known in the art. Sambrook et al., Molecular Cloning—A Laboratory Manual, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., (1989) and Current Protocols in Molecular Biology, F. M. Ausubel et al., eds., (Current Protocols, a joint venture between Greene Publishing Associates, Inc. and John Wiley & Sons, Inc., most recent Supplement).

Nucleic acids used to transfect cells with sequences coding for expression of the polypeptide of interest generally will be in the form of an expression vector including expression control sequences operatively linked to a nucleotide sequence coding for expression of the polypeptide. As used, the term “nucleotide sequence coding for expression of” a polypeptide refers to a sequence that, upon transcription and translation of mRNA, produces the polypeptide. This can include sequences containing, e.g., introns. As used herein, the term “expression control sequences” refers to nucleic acid sequences that regulate the expression of a nucleic acid sequence to which it is operatively linked. Expression control sequences are operatively linked to a nucleic acid sequence when the expression control sequences control and regulate the transcription and, as appropriate, translation of the nucleic acid sequence. Thus, expression control sequences can include appropriate promoters, enhancers, transcription terminators, a start codon (i.e., ATG) in front of a protein-encoding gene, splicing signals for introns, maintenance of the correct reading frame of that gene to permit proper translation of the mRNA, and stop codons.

Methods which are well known to those skilled in the art can be used to construct expression vectors containing the fluorescent indicator coding sequence and appropriate transcriptional/translational control signals. These methods include in vitro recombinant DNA techniques, synthetic techniques and in vivo recombination/genetic recombination. (See, for example, the techniques described in Maniatis, et al., Molecular Cloning A Laboratory Manual, Cold Spring Harbor Laboratory, N.Y., 1989).

Transformation of a host cell with recombinant DNA may be carried out by conventional techniques as are well known to those skilled in the art. Where the host is prokaryotic, such as E. coli, competent cells which are capable of DNA uptake can be prepared from cells harvested after exponential growth phase and subsequently treated by the CaCl₂ method by procedures well known in the art. Alternatively, MgCl₂ or RbCl can be used. Transformation can also be performed after forming a protoplast of the host cell or by electroporation.

When the host is a eukaryote, such methods of transfection of DNA as calcium phosphate co-precipitates, conventional mechanical procedures such as microinjection, electroporation, insertion of a plasmid encased in liposomes, or virus vectors may be used. Eukaryotic cells can also be cotransfected with DNA sequences encoding the fusion polypeptide of the invention, and a second foreign DNA molecule encoding a selectable phenotype, such as the herpes simplex thymidine kinase gene. Another method is to use a eukaryotic viral vector, such as simian virus 40 (SV40) or bovine papilloma virus, to transiently infect or transform eukaryotic cells and express the protein. (Eukaryotic Viral Vectors, Cold Spring Harbor Laboratory, Gluzman ed., 1982). Preferably, a eukaryotic host is utilized as the host cell as described herein.

Techniques for the isolation and purification of either microbially or eukaryotically expressed polypeptides of the invention may be by any conventional means such as, for example, preparative chromatographic separations and immunological separations such as those involving the use of monoclonal or polyclonal antibodies or antigen.

A variety of host-expression vector systems may be utilized to express fluorescent indicator coding sequence. These include but are not limited to microorganisms such as bacteria transformed with recombinant bacteriophage DNA, plasmid DNA or cosmid DNA expression vectors containing a fluorescent indicator coding sequence; yeast transformed with recombinant yeast expression vectors containing the fluorescent indicator coding sequence; plant cell systems infected with recombinant virus expression vectors (e.g., cauliflower mosaic virus, CaMV; tobacco mosaic virus, TMV) or transformed with recombinant plasmid expression vectors (e.g., Ti plasmid) containing a fluorescent indicator coding sequence; insect cell systems infected with recombinant virus expression vectors (e.g., baculovirus) containing a fluorescent indicator coding sequence; or animal cell systems infected with recombinant virus expression vectors (e.g., retroviruses, adenovirus, vaccinia virus) containing a fluorescent indicator coding sequence, or transformed animal cell systems engineered for stable expression.

Depending on the host/vector system utilized, any of a number of suitable transcription and translation elements, including constitutive and inducible promoters, transcription enhancer elements, transcription terminators, etc. may be used in the expression vector (see, e.g., Bitter, et al., Methods in Enzymology 153:516-544, 1987). For example, when cloning in bacterial systems, inducible promoters such as pL of bacteriophage λ, plac, ptrp, ptac (ptrp-lac hybrid promoter) and the like may be used. When cloning in mammalian cell systems, promoters derived from the genome of mammalian cells (e.g., metallothionein promoter) or from mammalian viruses (e.g., the retrovirus long terminal repeat; the adenovirus late promoter; the vaccinia virus 7.5K promoter) may be used. Promoters produced by recombinant DNA or synthetic techniques may also be used to provide for transcription of the inserted fluorescent indicator coding sequence.

In bacterial systems a number of expression vectors may be advantageously selected depending upon the use intended for the fluorescent indicator expressed. For example, when large quantities of the fluorescent indicator are to be produced, vectors which direct the expression of high levels of fusion protein products that are readily purified may be desirable. Those which are engineered to contain a cleavage site to aid in recovering fluorescent indicator are preferred.

In yeast, a number of vectors containing constitutive or inducible promoters may be used. For a review see, Current Protocols in Molecular Biology, Vol. 2, Ed. Ausubel, et al., Greene Publish. Assoc. & Wiley Interscience, Ch. 13, 1988; Grant, et al., Expression and Secretion Vectors for Yeast, in Methods in Enzymology, Eds. Wu & Grossman, 31987, Acad. Press, N.Y., Vol. 153, pp.516-544, 1987; Glover, DNA Cloning, Vol. II, IRL Press, Wash., D.C., Ch. 3, 1986; and Bitter, Heterologous Gene Expression in Yeast, Methods in Enzymology, Eds. Berger & Kimmel, Acad. Press, N.Y., Vol. 152, pp. 673-684, 1987; and The Molecular Biology of the Yeast Saccharomyces, Eds. Strathern et al., Cold Spring Harbor Press, Vols. I and II, 1982. A constitutive yeast promoter such as ADH or LEU2 or an inducible promoter such as GAL may be used (Cloning in Yeast, Ch. 3, R. Rothstein In: DNA Cloning Vol. 11, A Practical Approach, Ed. DM Glover, IRL Press, Wash., D.C., 1986). Alternatively, vectors may be used which promote integration of foreign DNA sequences into the yeast chromosome.

In cases where plant expression vectors are used, the expression of a fluorescent indicator coding sequence may be driven by any of a number of promoters. For example, viral promoters such as the 35S RNA and 19S RNA promoters of CaMV (Brisson, et al., Nature 310:511-514, 1984), or the coat protein promoter to TMV (Takamatsu, et al., EMBO J. 6:307-311, 1987) may be used; alternatively, plant promoters such as the small subunit of RUBISCO (Coruzzi, et al., 1984, EMBO J. 3:1671-1680; Broglie, et al., Science 224:838-843, 1984); or heat shock promoters, e.g., soybean hsp17.5-E or hsp17.3-B (Gurley, et al., Mol. Cell. Biol. 6:559-565, 1986) may be used. These constructs can be introduced into plant cells using Ti plasmids, Ri plasmids, plant virus vectors, direct DNA transformation, microinjection, electroporation, etc. For reviews of such techniques see, for example, Weissbach & Weissbach, Methods for Plant Molecular Biology, Academic Press, N.Y., Section VIII, pp. 421-463, 1988; and Grierson & Corey, Plant Molecular Biology, 2d Ed., Blackie, London, Ch. 7-9, 1988.

An alternative expression system which could be used to express fluorescent indicator is an insect system. In one such system, Autographa californica nuclear polyhedrosis virus (AcNPV) is used as a vector to express foreign genes. The virus grows in Spodoptera frugiperda cells. The fluorescent indicator coding sequence may be cloned into non-essential regions (for example, the polyhedrin gene) of the virus and placed under control of an AcNPV promoter (for example the polyhedrin promoter). Successful insertion of the fluorescent indicator coding sequence will result in inactivation of the polyhedrin gene and production of non-occluded recombinant virus (i.e., virus lacking the proteinaceous coat coded for by the polyhedrin gene). These recombinant viruses are then used to infect Spodoptera frugiperda cells in which the inserted gene is expressed, see Smith, et al., J. Viol. 46:584, 1983; Smith, U.S. Pat. No. 4,215,051.

Eukaryotic systems, and preferably mammalian expression systems, allow for proper post-translational modifications of expressed mammalian proteins to occur. Eukaryotic cells which possess the cellular machinery for proper processing of the primary transcript, glycosylation, phosphorylation, and, advantageously secretion of the gene product should be used as host cells for the expression of fluorescent indicator. Such host cell lines may include but are not limited to CHO, VERO, BHK, HeLa, COS, MDCK, Jurkat, HEK-293, and WI38.

Mammalian cell systems which utilize recombinant viruses or viral elements to direct expression may be engineered. For example, when using adenovirus expression vectors, the fluorescent indicator coding sequence may be ligated to an adenovirus transcription/translation control complex, e.g., the late promoter and tripartite leader sequence. This chimeric gene may then be inserted in the adenovirus genome by in vitro or in vivo recombination. Insertion in a non-essential region of the viral genome (e.g., region E1 or E3) will result in a recombinant virus that is viable and capable of expressing the fluorescent indicator in infected hosts (e.g., see Logan & Shenk, Proc. Natl. Acad. Sci. USA, 81: 3655-3659, 1984). Alternatively, the vaccinia virus 7.5K promoter may be used. (e.g., see, Mackett, et al., Proc. Natl. Acad. Sci. USA ,79: 7415-7419, 1982; Mackett, et al., J. Virol. 49: 857-864, 1984; Panicali, et al., Proc. Natl. Acad. Sci. USA 79: 4927-4931, 1982). Of particular interest are vectors based on bovine papilloma virus which have the ability to replicate as extrachromosomal elements (Sarver, et al., Mol. Cell. Biol. 1: 486, 1981). Shortly after entry of this DNA into mouse cells, the plasmid replicates to about 100 to 200 copies per cell. Transcription of the inserted cDNA does not require integration of the plasmid into the host's chromosome, thereby yielding a high level of expression. These vectors can be used for stable expression by including a selectable marker in the plasmid, such as the neo gene. Alternatively, the retroviral genome can be modified for use as a vector capable of introducing and directing the expression of the fluorescent indicator gene in host cells (Cone & Mulligan, Proc. Natl. Acad. Sci. USA, 81:6349-6353, 1984). High level expression may also be achieved using inducible promoters, including, but not limited to, the metallothionine IIA promoter and heat shock promoters.

For long-term, high-yield production of recombinant proteins, stable expression is preferred. Rather than using expression vectors which contain viral origins of replication, host cells can be transformed with the fluorescent indicator cDNA controlled by appropriate expression control elements (e.g., promoter, enhancer, sequences, transcription terminators, polyadenylation sites, etc.), and a selectable marker. The selectable marker in the recombinant plasmid confers resistance to the selection and allows cells to stably integrate the plasmid into their chromosomes and grow to form foci which in turn can be cloned and expanded into cell lines. For example, following the introduction of foreign DNA, engineered cells may be allowed to grow for 1-2 days in an enriched media, and then are switched to a selective media. A number of selection systems may be used, including but not limited to the herpes simplex virus thymidine kinase (Wigler, et al., Cell, 11: 223, 1977), hypoxanthine-guanine phosphoribosyltransferase (Szybalska & Szybalski, Proc. Natl. Acad. Sci. USA, 48:2026, 1962), and adenine phosphoribosyltransferase (Lowy, et al., Cell, 22: 817, 1980) genes can be employed in tk⁻, hgprt⁻ or aprt cells respectively. Also, antimetabolite resistance can be used as the basis of selection for dhfr, which confers resistance to methotrexate (Wigler, et al., Proc. Natl. Acad. Sci. USA, 77: 3567, 1980; O'Hare, et al., Proc. Natl. Acad. Sci. USA, 8: 1527, 1981); gpt, which confers resistance to mycophenolic acid (Mulligan & Berg, Proc. Natl. Acad. Sci. USA, 78: 2072, 1981; neo, which confers resistance to the aminoglycoside G-418 (Colberre-Garapin, et al., J. Mol. Biol., 150:1, 1981); and hygro, which confers resistance to hygromycin (Santerre, et al., Gene, 30: 147, 1984) genes. Recently, additional selectable genes have been described, namely trpB, which allows cells to utilize indole in place of tryptophan; hisD, which allows cells to utilize histinol in place of histidine (Hartman & Mulligan, Proc. Natl. Acad. Sci. USA, 85:8047, 1988); and ODC (ornithine decarboxylase) which confers resistance to the ornithine decarboxylase inhibitor, 2-(difluoromethyl)-DL-ornithine, DFMO (McConlogue L., In: Current Communications in Molecular Biology, Cold Spring Harbor Laboratory, ed., 1987).

DNA sequences encoding the fluorescence indicator polypeptide of the invention can be expressed in vitro by DNA transfer into a suitable host cell. “Host cells” are cells in which a vector can be propagated and its DNA expressed. The term also includes any progeny of the subject host cell. It is understood that all progeny may not be identical to the parental cell since there may be mutations that occur during replication. However, such progeny are included when the term “host cell” is used. Methods of stable transfer, in other words when the foreign DNA is continuously maintained in the host, are known in the art.

Recombinant fluorescent protein can be produced by expression of nucleic acid encoding the protein in prokaryotes, such as E. coli or in eukaryotes, such as yeast cells or mammalian cells. The fluorophore of Aeguorea-related fluorescent proteins results from cyclization and oxidation of residues 65-67.

The construct can also contain a tag to simplify isolation of the fluorescent indicator. For example, a polyhistidine tag of, e.g., six histidine residues, can be incorporated at the amino terminal end of the fluorescent protein. The polyhistidine tag allows convenient isolation of the protein in a single step by nickel-chelate chromatography.

In a preferred embodiment, the fluorescent indicator is a fusion protein produced by recombinant DNA technology in which a single polypeptide includes a donor moiety, a peptide linker moiety and an acceptor moiety. The donor moiety can be positioned at the amino-terminus relative to the acceptor moiety in the polypeptide. Such a fusion protein has the generalized structure: (amino terminus) donor fluorescent protein moiety—peptide linker moiety—acceptor fluorescent protein moiety (carboxy terminus). Alternatively, the donor moiety can be positioned at the carboxy-terminus relative to the acceptor moiety within the fusion protein. Such a fusion protein has the generalized structure: (amino terminus) acceptor fluorescent protein moiety—peptide linker moiety—donor fluorescent protein moiety (carboxy terminus). The invention also envisions fusion proteins that contain extra amino acid sequences at the amino and/or carboxy termini, for example, polyhistidine tags.

Thus, fluorescent indicators encoded by a recombinant nucleic acid include sequences coding for expression of a donor fluorescent protein moiety, an acceptor fluorescent protein moiety and a peptide linker moiety. The elements are selected so that upon expression into a fusion protein, the donor and acceptor moieties exhibit FRET when the donor moiety is excited. The recombinant nucleic acid can be incorporated into an expression vector comprising expression control sequences operatively linked to the recombinant nucleic acid. The expression vector can be adapted for function in prokaryotes or eukaryotes by inclusion of appropriate promoters, replication sequences, markers, etc.

The expression vector can be transfected into a host cell for expression of the recombinant nucleic acid. Host cells can be selected for high levels of expression in order to purify the fluorescent indicator fusion protein. E. coli is useful for this purpose. Alternatively, the host cell can be a prokaryotic or eukaryotic cell selected to study the activity of an enzyme produced by the cell. In this case, the linker peptide is selected to include an amino acid sequence recognized by the protease. The cell can be, e.g., a cultured cell or a cell in vivo.

A primary advantage of fluorescent indicator fusion proteins is that they are prepared by normal protein biosynthesis, thus completely avoiding organic synthesis and the requirement for customized unnatural amino acid analogs. The constructs can be expressed in E. coli in large scale for in vitro assays. Purification from bacteria is simplified when the sequences include polyhistidine tags for one-step purification by nickel-chelate chromatography. Alternatively, the substrates can be expressed directly in a desired host cell for assays in situ.

In another embodiment, the invention provides a transgenic non-human animal that expresses a nucleic acid sequence which encodes the fluorescent indicator.

The “non-human animals” of the invention comprise any non-human animal having nucleic acid sequence which encodes a fluorescent indicator. Such non-human animals include vertebrates such as rodents, non-human primates, sheep, dog, cow, pig, amphibians, and reptiles. Preferred non-human animals are selected from the rodent family including rat and mouse, most preferably mouse. The “transgenic non-human animals” of the invention are produced by introducing “transgenes” into the germline of the non-human animal. Embryonal target cells at various developmental stages can be used to introduce transgenes. Different methods are used depending on the stage of development of the embryonal target cell. The zygote is the best target for micro-injection. In the mouse, the male pronucleus reaches the size of approximately 20 micrometers in diameter which allows reproducible injection of 1-2 pl of DNA solution. The use of zygotes as a target for gene transfer has a major advantage in that in most cases the injected DNA will be incorporated into the host gene before the first cleavage (Brinster et al., Proc. Natl. Acad. Sci. USA 82:4438-4442, 1985). As a consequence, all cells of the transgenic non-human animal will carry the incorporated transgene. This will in general also be reflected in the efficient transmission of the transgene to offspring of the founder since 50% of the germ cells will harbor the transgene. Microinjection of zygotes is the preferred method for incorporating transgenes in practicing the invention.

The term “transgenic” is used to describe an animal which includes exogenous genetic material within all of its cells. A “transgenic” animal can be produced by cross-breeding two chimeric animals which include exogenous genetic material within cells used in reproduction. Twenty-five percent of the resulting offspring will be transgenic i.e., animals which include the exogenous genetic material within all of their cells in both alleles. 50% of the resulting animals will include the exogenous genetic material within one allele and 25% will include no exogenous genetic material.

Retroviral infection can also be used to introduce transgene into a non-human animal. The developing non-human embryo can be cultured in vitro to the blastocyst stage. During this time, the blastomeres can be targets for retro viral infection (Jaenich, R., Proc. Natl. Acad. Sci USA 73:1260-1264, 1976). Efficient infection of the blastomeres is obtained by enzymatic treatment to remove the zona pellucida (Hogan, et al. (1986) in Manipulating the Mouse Embryo, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). The viral vector system used to introduce the transgene is typically a replication-defective retro virus carrying the transgene (Jahner, et al., Proc. Natl. Acad. Sci. USA 82:6927-6931, 1985; Van der Putten, et al., Proc. Natl. Acad. Sci USA 82:6148-6152, 1985). Transfection is easily and efficiently obtained by culturing the blastomeres on a monolayer of virus-producing cells (Van der Putten, supra; Stewart, et al., EMBO J. 6:383-388, 1987). Alternatively, infection can be performed at a later stage. Virus or virus-producing cells can be injected into the blastocoele (D. Jahner et al., Nature 298:623-628, 1982). Most of the founders will be mosaic for the transgene since incorporation occurs only in a subset of the cells which formed the transgenic nonhuman animal. Further, the founder may contain various retro viral insertions of the transgene at different positions in the genome which generally will segregate in the offspring. In addition, it is also possible to introduce transgenes into the germ line, albeit with low efficiency, by intrauterine retro viral infection of the midgestation embryo (D. Jahner et al., supra).

A third type of target cell for transgene introduction is the embryonal stem cell (ES). ES cells are obtained from pre-implantation embryos cultured in vitro and fused with embryos (M. J. Evans et al. Nature 292:154-156, 1981; M. O. Bradley et al., Nature 309: 255-258, 1984; Gossler, et al., Proc. Natl. Acad. Sci USA 83: 9065-9069, 1986; and Robertson et al., Nature 322:445-448, 1986). Transgenes can be efficiently introduced into the ES cells by DNA transfection or by retro virus-mediated transduction. Such transformed ES cells can thereafter be combined with blastocysts from a nonhuman animal. The ES cells thereafter colonize the embryo and contribute to the germ line of the resulting chimeric animal. (For review see Jaenisch, R., Science 240: 1468-1474, 1988).

“Transformed” means a cell into which (or into an ancestor of which) has been introduced, by means of recombinant nucleic acid techniques, a heterologous nucleic acid molecule. “Heterologous” refers to a nucleic acid sequence that either originates from another species or is modified from either its original form or the form primarily expressed in the cell.

“Transgene” means any piece of DNA which is inserted by artifice into a cell, and becomes part of the genome of the organism (i.e., either stably integrated or as a stable extrachromosomal element) which develops from that cell. Such a transgene may include a gene which is partly or entirely heterologous (i.e., foreign) to the transgenic organism, or may represent a gene homologous to an endogenous gene of the organism. Included within this definition is a transgene created by the providing of an RNA sequence which is transcribed into DNA and then incorporated into the genome. The transgenes of the invention include DNA sequences which encode which encodes the fluorescent indicator which may be expressed in a transgenic non-human animal. The term “transgenic” as used herein additionally includes any organism whose genome has been altered by in vitro manipulation of the early embryo or fertilized egg or by any transgenic technology to induce a specific gene knockout. The term “gene knockout” as used herein, refers to the targeted disruption of a gene in vivo with complete loss of function that has been achieved by any transgenic technology familiar to those in the art. In one embodiment, transgenic animals having gene knockouts are those in which the target gene has been rendered nonfunctional by an insertion targeted to the gene to be rendered non-functional by homologous recombination. As used herein, the term “transgenic” includes any transgenic technology familiar to those in the art which can produce an organism carrying an introduced transgene or one in which an endogenous gene has been rendered non-functional or “knocked out.”

Without further elaboration, it is believed that one skilled in the art can, based on the description herein, utilize the present invention to its fullest extent. All patents and publications cited herein are hereby incorporated by reference.

A fluorescent indicator for Ca²⁺ was produced by sandwiching CaM-M13 fusion, described in Porumb, T., et al., Prot. Engineering 7:109-115 (1994), between a blue (P4-3) and a green (S65T) GFP mutant, as illustrated in FIG. 2 a. The chimeric cDNA was cloned at BamHI/EcoRI sites of pRSETB (Invitrogen), HIS is the amino-terminal tag peptide containing the polyhistidine sequence and XCaM is Xenopus calmodulin. Chimeric proteins incorporating a polyhistidine tag were expressed in Escherichia coli, purified by nickel-chelate and size-exclusion chromatography, and their fluorescence characterized. Referring to FIG. 2 a, the fluorescent CaM-based indicator (“cameleon-1”) readily changes emission color by retracting and extending a long tongue (M13) into and out of the mouth of the CaM.

The amino acid composition of the boundary regions between the CaM-M13 hybrid and GFPs can be important to optimize protein folding and the Ca²⁺-sensitivity of FRET. One particularly sensitive indicator is shown in FIG. 2 b. Referring to FIG. 2 b, the amino acid and nucleotide sequences of the boundaries between P4-3 and CaM and between M13 and S65T in cameleon-1 are shown. Cameleon-1 has an 11 amino acid deletion at the C-terminus of P4-3 and a 5 amino acid deletion at the C-terminus of M13. Two restriction sites SphI and SacI are underlined, which were utilized to connect the genes of P4-3 and S65T to the CaM-M13 gene, respectively. To facilitate the folding of the GFP that is fused to any other protein, a few glycine residues are usually inserted into the boundary. See, for example, Porumb, T., et al., Prot. Engineering 7:109-115 (1994). Further, glycine residues were not introduced so that P4-3 and S65T were fused rigidly to the CaM and M13, respectively. The rigid fusion leads to more effective transduction of the conformational change of CaM-M13, causing a greater change in FRET efficiency.

The fluorescent indicator was efficiently expressed and folded in bacteria and increased its ratio of UV-excited 510 nm to 445 nm emissions by 70% upon binding Ca²⁺, as shown in FIG. 3. The emission spectra of cameleon-1 were measured in 100 mM KCl, 20 mM MOPS, 1 mM EDTA, KOH to pH 7.20, before and after addition of 2 mM CaCl₂ to give 1 mM free Ca²⁺. The Ca²⁺ binding to EDTA caused a local acidification of the solution, and a small fraction of the protein was denatured. Thus the spectrum after the Ca²⁺ addition dropped down slightly (compare with FIG. 4 a). The decrease in blue and increase in green emission indicated that Ca²⁺ increased the efficiency of FRET from P4-3 to S65T, consistent with the expected decrease in distance between the two ends of the protein. The Ca²⁺ response was fully reversible upon chelation of Ca²⁺.

The Ca²⁺-specificity of the response of cameleon-1 was examined. Mg²⁺, pH, and ionic strength did not alter the emission spectra of either the Ca²⁺-saturated and Ca-unsaturated forms. The emission spectra of saturated and unsaturated cameleon-1 were also not affected by hydrophobic proteins such as bovine serum albumin. Isolated CaM saturated with Ca²⁺ typically becomes sticky with hydrophobic amino acids exposed to the surface. The CaM in cameleon-1, on the other hand, appears to interact preferentially with its intramolecularly-adjacent M13 peptide. The self-contained nature of the system minimizes the possibility that the protein might interact with endogenous CaM-binding sequences in eukaryotic cells.

The Ca²⁺ binding behavior of cameleon-1 was examined. The CaM-M13 hybrid protein without GFPs displayed a biphasic Ca²⁺ binding with two dissociation constants (80 nM and 2 μM). See, Porumb, T., et al., Prot. Engineering 7:109-115 (1994). Titration experiments revealed that the emission ratio of cameleon-1 has a biphasic Ca²⁺ dependency, as shown in FIGS. 4 a and 4 b. FIG. 4 a shows the change in emission spectrum of cameleon-1 on titration with Ca²⁺ when excited at 380 nm. The titration was done using KHHEDTA/KCaHEDTA solutions at pH 7.47. For clarity only two intermediate concentrations of Ca²⁺ are shown.

FIG. 4 b shows Ca²⁺ titration curves of cameleon-1 (open circles) and cameleon-1/E104Q (solid circles). Data points were from 4 independent experiments at different pH using Ca²⁺/EGTA and C²⁺/HEEDTA systems for each protein. In each experiment, the emission ratio (510/445 nm) change was normalized to the value of full saturation with Ca²⁺, which increased by 60-75% over the value of zero Ca²⁺. The data of cameleon-1/E104Q were fitted to a four parameter logistic function curve (dotted line). The data of cameleon-1 were analysed using a linear combination of 2 four parameter logistic function fits (solid line). The apparent dissociation constants (K′_(d)s) for cameleon-1 were 68 nM and 11 μM, and the Hill coefficients were 1.8 and 1.0, respectively. The binding curve can be used to quantify the concentration of Ca²⁺ present in the sample. Because of simulated negative cooperativity, cameleon-1 covers a very wide range of Ca²⁺) concentration, from <10⁻⁷ to ˜10⁻³ M.

The affinity of the CaM binding protein moiety can be modified. Many site-directed mutations have been studied for their effects on the Ca²⁺ binding and C²⁺-induced conformational changes of CaM. See, Maune, J. F., et al., J. Biol. Chem. 267:5286-5295 (1992), and Gao, Z. H., et al., J. Biol. Chem. 268:20096-20104 (1993). For example, a mutant chimera protein with the conserved bidentate glutamic acid at position 12 of the third Ca²⁺ binding loop of the CaM mutated to glutamine (cameleon-1/E104Q) was constructed. The mutation eliminated the high-affinity response of cameleon-1, as indicated in FIG. 4 b, (solid circles). Cameleon-1/E104Q showed a monophasic response (K′d, 4.4 μM; Hill coefficient, 0.76), which corresponds closely to the low affinity component of the cameleon-1 FRET response. Other modifications of CaM can be made to tune the Ca²⁺ affinities for particular applications of the fluorescent indicator.

HeLa cells were transfected with the recombinant plasmid (cameleon-1/pCDNA3) to determine whether cameleon-1 can work as a Ca²⁺ indicator in live cells. When the cells were excited with UV, however, the fluorescence of cameleon-1 was hardly detectable, mainly because of the dim fluorescence of the P4-3 component. Expression and folding of GFP at 37° C. in mammalian cells was improved by introducing mammalian codon bias into the cDNA and mutating Phe64 to Leu, as in the commercially available construct “EGFP” (Clonetech), which encodes F64L/S65T with mammalian codons. The same changes were introduced into P4-3 (Y66H/Y145F), which did not change its final fluorescence properties but did improve expression in the HeLa cells. The improved blue mutant (“EBFP”) and EGFP substituted P4-3 and S65T, respectively, in cameleon-1 to make cameleon-2, shown in FIG. 5, where Kz is Kozak's consensus sequence (M. Kozak, J. Cell Biol. 108:229-241 (1989). The nucleotide sequence (SEQ ID NO:1) and amino acid sequence (SEQ ID NO:2) of cameleon-2 are shown in FIG. 7.

The E104Q mutation which afforded low Ca²⁺ affinity in cameleon-1 was also introduced into cameleon-2; the resulting chimera protein (cameleon-2/E104Q) cameleon-3, also shown in FIG. 5. The nucleotide sequence (SEQ ID NO:5) and amino acid sequence (SEQ ID NO:6) of cameleon-3 are shown in FIG. 9.

Significant emission signals were observed from both the EBFP and EGFP in the cells expressing cameleon-2 or cameleon-3. Referring to FIG. 6 a, fluorescence of cameleon-2 localized to the cytosol of HeLa cells. The fluorescence was uniformly distributed in the cytosolic compartment but excluded from the nucleus, as expected for a protein of 74 kDa without localization sequences or targeting signals. The image was taken using a 330WB80 (excitation filter) and a 535DF45 (emission filter). The bar is 10 μm.

FIG. 6 b shows time courses of the spatially averaged green:blue emission ratios from two individual HeLa cells expressing cameleon-2. The two cells shown in FIG. 6 a were excited by UV (330WB80) and monitored every 15 seconds by digital imaging microscopy. See, for example, Tsien, R. Y. & Harootunian, A. T. Cell Calcium, 11:93-109 (1990). The emission bands (440DF40 and 535DF45) over the cytoplasmic regions were alternately sampled. In situ calibration was performed for each of the cells. The pre-stimulation ratio (arrowhead) was assumed to be 50 nM, and R_(max) (arrow) the value after saturation with Ca²⁺. The calculated values for pcas 7, 6, 5, 4 are indicated by horizontal bars on the right side of the panel.

Elevation of cytosolic Ca²⁺ concentration by saturating doses of ATP (as a purinergic agonist) and histamine produced significant increases in the emission ratio. Blockage of the histamine receptor by the antagonist cyproheptadine caused a rapid decrease in ratio, indicating the reversible behavior of the indicator. Addition of ionomycin followed by a high concentration (15 mM) of extracellular Ca²⁺ gave a large increase of the ratio (70-80% increase of the initial ratio value), which should correspond to the maximal ratio R_(max). Assuming the lowest ratio observed before stimulation represents the R_(min), calibration for free Ca²⁺ concentration can be performed. See, Adams, S. R., et al., in Fluorescent and Luminescent Probes for Biological Activity (ed. Mason, W. T.) (Academic Press, 1993).

By contrast, cameleon-3, which lacks the high affinity component of Ca²⁺ binding to CaM, did not detect the changes in cytosolic Ca²⁺ concentration signals due to ATP or histamine, but gave a similar R_(max) in response to ionomycin and 20 mM extracellular Ca²⁺. Cameleon-3 is less sensitive to and buffers cytosolic Ca²⁺ to a lesser extent than does cameleon-2. The probable high Ca²⁺ dissociation rate of cameleon-3 is advantageous for tracking rapid Ca²⁺ release kinetics. The in vitro study revealed that the cameleon indicators show a relatively fast cellular response to Ca²⁺ concentration changes.

Addition of a nuclear localization sequence to cameleon-2 yielded a Ca²⁺ indicator, cameleon-2nu, shown in FIG. 5. The nucleotide sequence (SEQ ID NO:3) and amino acid sequence (SEQ ID NO:4) of cameleon-2nu are shown in FIG. 8. The fluorescence of cameleon-2nu was localized to nuclei, as depicted in FIG. 6 c. The time course of Ca²⁺ concentrations in nuclei was followed (FIG. 6 d) and was similar to the results obtained in the cytosol (FIG. 6 b).

Agonist-induced changes in the free Ca²⁺ concentration inside the endoplasmic reticulum in intact cells were similarly monitored. The low-affinity indicator cameleon-3 was engineered to reside in the lumen of endoplasmic reticulum (ER) (cameleon-3er) (FIG. 5) by addition of a localization sequence at the amino terminus and a KDEL signal for ER retention at the carboxy terminus of the fluorescent indicator. The nucleotide sequence (SEQ ID NO:7) and amino acid sequence of cameleon-3er (SEQ ID NO:8) are shown in FIG. 10. Reticular patterns of fluorescence were seen in HeLa cells expressing the protein. FIG. 6 e is a digital fluorescence image of cameleon-3er in transfected HeLa cells. The image was obtained with a cooled CCD camera system with a 480DF30 (excitation filter) and a 535DF45 (emission filter). The bar is 10 μm.

FIG. 6 f shows a time course of the average Ca²⁺ _(er) concentration of four cells obtained with a video-rate confocal microscope. Digital fluorescence images were the result of simultaneous acquisition of two confocal single-wavelength emission images at 450 nm (65 nm bandpass) and 535 nm (45 nm bandpass). After background subtraction, the ratio of the long wavelength image over the short wavelength one was calculated. Cells were illuminated for 66 msec (2 frames) for each time point. The interference filters and dichroics, and the sensitivity of the detectors are different between the CCD and confocal microscope systems. Therefore, the ratios obtained in the two systems differ quantitatively.

The pre-stimulus Ca²⁺ _(er) concentration reported by cameleon-3er was consistently higher than cytosolic or nuclear Ca²⁺ concentrations reported by cameleon-2 and cameleon-2nu, respectively. Histamine reproducibly decreased the Ca²⁺ _(er) concentration in all of 15 cells, whereas it always increased the cytosolic and nuclear Ca²⁺ concentrations. Receptor blockade by cyproheptadine reversed the decrease Ca²⁺ _(er) concentration, indicating refill of the Ca²⁺ pools. In FIG. 6 f, the ratio did not reach the value of the resting state, whereas complete reversion was observed in five other experiments. Addition of ionomycin and 20 mM extracellular Ca²⁺ increased Ca²⁺ _(er) concentration to a saturating value above the starting level. Changes in Ca²⁺ _(er) concentration were generally slower than those of the cytosol and nuclei.

The targetability of the fluorescent indicators can permit Ca²⁺ measurements at previously inaccessible sites such as the immediate vicinity of synaptic vesicles or Ca²⁺ channels, and in genetically tractable organisms that do not load well with esters of small-molecule indicators.

The Examples described below are illustrative of the disclosed method; however, many alternatives, modifications and variations will be clear to those skilled in the art.

EXAMPLES

Gene Construction

The cDNA of the GFP mutant P4-3 was amplified by PCR with a sense primer containing a BamHI site, and a reverse primer containing an SphI site and eliminating the GFP stop codon. See, for example, Heim, R. & Tsien, R. Y. Current Biol. 6:178-182 (1996). Likewise, the cDNA of S65T was amplified with a SacI site and an EcoRI site introduced to the 5′ and 3′ ends of the gene, respectively. Two restriction sites (SphI and SacI) were introduced by PCR into 5′ and 3′ ends of the CaM-M13 gene, respectively, using the pHY1 as a template. See, Porumb, T., et al. Prot. Engineering 7:109-115 (1994). All the amplification reactions were done by Pfu polymerase (Stratagene). The restricted products were ligated and cloned in-frame into the BamHI/EcoRI sites of pRSETB (Invitrogen). The modifications of the boundary regions between P4-3 and CaM and between M13 and S65T were performed by PCR or by a combined use of restriction enzymes, Klenow fragment of DNA polymerase I, T4 DNA polymerase, mung bean exonuclease, and T4 DNA ligase as described, for example, in Molecular Cloning, A Laboratory Manual (eds. Sambrook, J., Fritsch, E. F. & Maniatis, T.) (CSH Laboratory Press, 1989). The phEGFP plasmid was commercially available from Clontech. Two amino acid substitutions (Y66H and Y145F) were made in hEGFP to construct EBFP. oligonucleotide-directed mutageneses were carried out using the Muta-Gene Phagemid in vitro kit (Bio-Rad) at the codons for Y66H and Y145F of EGFP, and for E104Q of the calmodulin. The 5′ end of the EBFP gene was modified by PCR to have a HindIII restriction site followed by a Kozak's consensus sequence (ACCGCC-ATG). The HindIII/EcoRI fragment encoding the entire chimeric protein was subcloned in the mammalian expression vector pCDNA3 (Invitrogen).

For cameleon-2nu, the cameleon-2 DNA was extended by PCR at the 3′ end with the sequence encoding the nuclear localization sequence (PKKKRKVEDP). See, Forbes, D. J. Ann. Rev. Cell Biol. 8:495-527 (1992). Cameleon-3er DNA was likewise obtained by extending the cameleon-3 DNA at the 5′ end with the sequence encoding the localization sequence peptide from calreticulin (MLLPVPLLLGLLGLAAAD), and at the 3′ end with the sequence encoding the ER retention signal (KDEL). See, Kendall, J. M. et al., Biochem. Biophys. Res. Commun. 189:1008-1016 (1992).

Protein Expression and Spectroscopy

The expression of chimera proteins in bacteria was performed using the T7 expresson system (pRSETB/JM109(DE3)). Cultures were grown at room temperature, and protein expression was induced by isopropyl B-D-thiogalactoside. Cells were lysed by a French press. The polyhistidine-tagged chimera proteins were purified from the cleared lysates on nickel-chelate columns (Qiagen). The protein samples in the eluates were concentrated by Centricon 30 (Amicon), and were further purified by gel-filtration column to remove abortive chimera proteins which resulted from proteolysis or misfolding. Emission spectra of the purified proteins were measured using a fluorometer (Spex Industries, Edison, N.J.) at excitation 380 nm.

Ca²⁺ Titration and Calibration

The titration experiments were performed by the “pH-metric method” as described in Grzegorz, G., et al., J. Biol. Chem. 260:3440-3450 (1985). In situ calibration for cytosolic Ca²⁺ concentration utilized the equation: [Ca²⁺]c=K′_(d)((R−R_(min))/(R_(max)−R))^((1/nH)) where K′_(d) is the apparent dissociation constant corresponding to the Ca²⁺ concentration at which R is midway between R_(max) and R_(min), and nH is the Hill coefficient. Imaging

Two to five days after the cDNA transfection with lipofectin (Gibco BRL), the cells were imaged on a Zeiss Axiovert microscope with a cooled CCD camera (Photometrics, Tucson, Ariz.) interfaced to a personal computer. The program MetaFluor 2.75 (Universal Imaging) was used for controlling data acquisition and analysis. Dual-emission ratio imaging was carried out by manually switching the two emission filters (440DF40 for EBFP, 535DF45 for EGFP) in front of a single imaging camera. The excitation filter (330WB80) was used with a 420DRLP dichroic mirror. Digital fluorescence imaging with a video-rate confocal microscope was performed as described in Tsien, R. Y. & Backskai, B. J. Handbook of Biological Confocal Microscopy (ed. Pawley, J. B.) (Plenum Press, New York, 1995) p. 459-478. Cells were illuminated with wide band UV (351-364 nm) from an Ar⁺ ion laser. The primary dichroic (380DRLP) reflects UV and transmits light emitted from the specimen, which is subsequently split by a secondary dichroic (505DRLP) into two broad bands: EBFP emission (450DF65) and EGFP emission (535DF45), and counted by photomultiplier tubes.

From the above description, the essential characteristics of the present invention can be ascertained. Without departing from the spirit and scope thereof, various changes and modifications of the invention can be made to adapt it to various usages and conditions. Thus, other embodiments are also within the claims. 

1. A fluorescent indicator comprising: a binding protein moiety having an analyte-binding region which binds an analyte and causes the indicator to change conformation upon exposure to the analyte; a donor fluorescent protein moiety covalently coupled to the binding protein moiety; and an acceptor fluorescent protein moiety covalently coupled to the binding protein moiety, wherein the donor moiety and the acceptor moiety change position relative to each other when the analyte binds to the analyte-binding region, altering fluorescence resonance energy transfer between the donor moiety and the acceptor moiety when the donor moiety is excited.
 2. The indicator of claim 1, wherein the donor fluorescent protein moiety and the acceptor fluorescent protein moiety are Aequorea-related fluorescent protein moieties.
 3. The indicator of claim 2, wherein the indicator further includes a target peptide moiety and a linker moiety that covalently couples the binding protein and the target peptide moiety, and the binding protein moiety further includes a peptide-binding region for binding the target peptide moiety.
 4. The indicator of claim 3, wherein the indicator further comprises a localization sequence.
 5. The indicator of claim 4, wherein the localization sequence is a nuclear localization sequence, an endoplasmic reticulum localization sequence, a peroxisome localization sequence, a mitochondrial localization sequence, or a localized protein.
 6. The indicator of claim 3, wherein the donor fluorescent protein moiety is covalently coupled to the binding protein moiety and the acceptor fluorescent protein moiety is covalently coupled to the target peptide moiety.
 7. The indicator of claim 3, wherein the linker moiety is a peptide moiety.
 8. The indicator of claim 7, wherein the linker moiety includes between about 1 amino acid residue and about 30 amino acid residues.
 9. The indicator of claim 7, wherein the indicator is a single polypeptide.
 10. The indicator of claim 9, wherein one of the donor fluorescent protein moiety or the acceptor fluorescent protein moiety is covalently coupled to the carboxy terminus of the single polypeptide and the other of the donor fluorescent protein moiety or the acceptor fluorescent protein moiety is covalently coupled to the amino terminus of the single polypeptide.
 11. The indicator of claim 2, wherein the binding protein moiety is calmodulin, a calmodulin-related protein moiety, cGMP-dependent protein kinase, a steroid hormone receptor, a ligand binding domain of a steroid hormone receptor, protein kinase C, inositol-1,4,5-triphosphate receptor, or recoverin.
 12. The indicator of claim 3, wherein the binding protein moiety is calmodulin or a calmodulin-related protein moiety.
 13. The indicator of claim 12, wherein the donor fluorescent protein moiety is P4-3, EBFP, or W1B, and the acceptor fluorescent protein moiety is S65T, EGFP, or 10c.
 14. The indicator of claim 12, wherein the target peptide moiety is a calmodulin-binding domain of skMLCKp, smMLCK, CaMKII, Caldesmon, Calspermin, phosphofructokinase calcineurin, phosphorylase kinase, Ca2+-ATPase 59 kDa PDE, 60 kDa PDE, nitric oxide synthase, type I adenylyl cylclase, Bordetella pertussis adenylyl cyclase, Neuromodulin, Spectrin, MARCKS, F52, β-Adducin, HSP90a, HIV-1 gp160, BBMHBI, Dilute MHC, Mastoparan, Melittin, Glucagon, Secretin, VIP, GIP, or Model Peptide CBP2.
 15. The indicator of claim 14, wherein the target peptide moiety is the calmodulin-binding domain of skMLCK.
 16. The indicator of claim 15, wherein the linker moiety is -Gly-Gly-.
 17. A fluorescent indicator comprising: a target peptide moiety; a binding protein moiety having an analyte-binding region which binds an analyte and causes the indicator to change conformation upon exposure to the analyte, and a peptide-binding region for binding the target peptide moiety; a linker moiety that covalently couples the binding protein and the target peptide moiety and is a peptide moiety; a donor fluorescent protein moiety covalently coupled to the binding protein moiety; and an acceptor fluorescent protein moiety covalently coupled to the target peptide moiety, wherein the donor moiety and the acceptor moiety change position relative to each other when the analyte binds to the analyte-binding region, altering fluorescence resonance energy transfer between the donor moiety and the acceptor moiety when the donor moiety is excited, and the indicator is a single polypeptide.
 18. The indicator of claim 17, wherein the donor fluorescent protein moiety and the acceptor fluorescent protein moiety are Aequorea-related fluorescent protein moieties.
 19. The indicator of claim 18, wherein the donor fluorescent protein moiety is P4-3, EBFP, or W1B, and the acceptor fluorescent protein moiety is S65T, EGFP, or 10c.
 20. The indicator of claim 19, wherein the binding protein moiety is calmodulin or a calmodulin-related protein moiety.
 21. The indicator of claim 20, wherein the target peptide moiety is M13.
 22. The indicator of claim 21, wherein one of the donor fluorescent protein moiety or the acceptor fluorescent protein moiety is located at the carboxy terminus of the single polypeptide and the other of the donor fluorescent protein moiety or the acceptor fluorescent protein moiety is located at the amino terminus of the single polypeptide.
 23. The indicator of claim 17, wherein the indicator further comprises a nuclear localization signal, an endoplasmic reticulum localization signal, a peroxisome localization signal, a mitochondrial localization signal, or a localized protein.
 24. A method for determining the concentration of an analyte in a sample comprising: contacting the sample with a fluorescent indicator comprising a binding protein moiety having an analyte-binding region which binds an analyte and causes the indicator to change conformation upon exposure to the analyte, a donor fluorescent protein moiety covalently coupled to the binding protein moiety, and an acceptor fluorescent protein moiety covalently coupled to the binding protein moiety, wherein the donor moiety and the acceptor moiety change position relative to each other when the analyte binds to the analyte-binding region, altering fluorescence resonance energy transfer between the donor moiety and the acceptor moiety when the donor moiety is excited; exciting the donor moiety; and determining the degree of fluorescence resonance energy transfer in the sample corresponding to the concentration of the analyte in the sample.
 25. The method of claim 24, wherein the step of determining the degree of fluorescence resonance energy transfer in the sample comprises measuring light emitted the acceptor fluorescent protein moiety.
 26. The method of claim 24, wherein determining the degree of fluorescence resonance energy transfer in the sample comprises measuring light emitted from the donor fluorescent protein moiety, measuring light emitted from the acceptor fluorescent protein moiety, and calculating a ratio of the light emitted from the donor fluorescent protein moiety and the light emitted from the acceptor fluorescent protein moiety.
 27. The method of claim 24, wherein the step of determining the degree of fluorescence resonance energy transfer in the sample comprises measuring the excited state lifetime of the donor moiety.
 28. The method of claim 26, further comprising the steps of determining the concentration of the analyte at a first time after contacting the sample with the fluorescence indicator, determining the concentration of the analyte at a second time after contacting the sample with the fluorescence indicator, and calculating the difference in the concentration of the analyte at the first time and the second time, whereby the difference in the concentration of the analyte in the sample reflects a change in concentration of the analyte present in the sample.
 29. The method of claim 28, further comprising the step of contacting the sample with a compound between the first time and the second time, whereby a difference in the concentration of the analyte in the sample between the first time and the second time indicates that the compound alters the presence of the analyte.
 30. The method of claim 24, wherein the sample comprises an intact cell and the contacting step comprises incorporating the fluorescent indicator into the cell.
 31. The method of claim 30, wherein step of incorporating the fluorescent indicator into the cell includes transfecting the cell with an expression vector comprising expression control sequences operably linked to a nucleic acid sequence coding for the expression of the fluorescent indicator.
 32. The method of claim 24, wherein the analyte is calcium.
 33. The method of claim 24, wherein the donor fluorescent protein moiety and the acceptor fluorescent protein moiety are Aequorea-related fluorescent protein moieties.
 34. The method of claim 33, wherein the indicator further includes a target peptide moiety and a linker moiety that covalently couples the binding protein and the target peptide moiety and is a peptide moiety, and the binding protein moiety further includes a peptide-binding region for binding a target peptide moiety.
 35. The method of claim 34, wherein the indicator is a single polypeptide.
 36. The method of claim 24, wherein the binding protein moiety is calmodulin or a calmodulin-related protein moiety, the donor fluorescent protein moiety is P4-3, EBFP, or W1B, the acceptor fluorescent protein moiety is S65T, EGFP, or 10c, and the target peptide moiety is M13.
 37. The method of claim 36, wherein the indicator further comprises a localization sequence. 